1. Field of the Disclosure
This disclosure relates to the testing of underground formations or reservoirs. More particularly, this disclosure relates to a method of reducing formation damage due to invasion of brine during drilling and/or hydraulic fracturing and for making more reliable estimates of formation permeability using prior art methods and apparatus.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or by rotating the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional down-hole instruments, known as logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine the formation geology and formation fluid conditions during the drilling operations.
Drilling fluid (commonly known as the “mud” or “drilling mud”) is pumped into the drill pipe to rotate the drill motor, provide lubrication to various members of the drill string including the drill bit and to remove cuttings produced by the drill bit. The drill pipe is rotated by a prime mover, such as a motor, to facilitate directional drilling and to drill vertical boreholes. The drill bit is typically coupled to a bearing assembly having a drive shaft, which in turn rotates the drill bit attached thereto. Radial and axial bearings in the bearing assembly provide support to the radial and axial forces of the drill bit.
Boreholes are usually drilled along predetermined paths and the drilling of a typical borehole proceeds through various formations. The drilling operator typically controls the surface-controlled drilling parameters, such as the weight on bit, drilling fluid flow through the drill pipe, the drill string rotational speed and the density and viscosity of the drilling fluid to optimize the drilling operations. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to optimize the drilling operations. For drilling a borehole in a virgin region, the operator typically has seismic survey plots which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator also has information about the previously drilled boreholes in the same formation.
Typically, the information provided to the operator during drilling includes borehole pressure and temperature and drilling parameters, such as Weight-On-Bit (WOB), rotational speed of the drill bit and/or the drill string, and the drilling fluid flow rate. In some cases, the drilling operator also is provided selected information about the bottom hole assembly condition (parameters), such as torque, mud motor differential pressure, torque, bit bounce and whirl etc.
Downhole sensor data are typically processed downhole to some extent and telemetered uphole by sending a signal through the drill string, or by mud-pulse telemetry which is transmitting pressure pulses through the circulating drilling fluid. Although mud-pulse telemetry is more commonly used, such a system is capable of transmitting only a few (1-4) bits of information per second. Due to such a low transmission rate, the trend in the industry has been to attempt to process greater amounts of data downhole and transmit selected computed results or “answers” uphole for use by the driller for controlling the drilling operations.
Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. Despite the advances in data acquisition during drilling using the MWD systems, it is often necessary to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after the well has been drilled, the hydrocarbon zones are often tested with other test equipment.
A problem commonly encountered with prior art devices is due to the invasion of the formation by borehole fluids. It is common practice during drilling operations to maintain the borehole fluid pressure slightly above the expected formation fluid pressure. By maintaining this overbalanced condition, the risk of blowouts is reduced. However, with this overbalanced condition, there is a likelihood of the borehole fluid invading the formation. When the borehole fluid is a water-based mud, the invasion of the formation by water can cause formation damage as well as errors in the formation evaluation.
This is illustrated in FIG. 5 where a number of grains 501 of the formation are shown. The formation itself may include a hydrocarbon such as oil or gas, denoted by 505. Many of the common minerals that make up earth formations are preferentially wetted by water. This is illustrated by the water coating 503 surrounding the grains. The water coating may form a continuous film 521 around the grains, impeding the flow of hydrocarbons 505 in the pore spaces such as 509 between the grains towards the borehole (direction indicated by 511). In this regards, it is useful to review certain definitions of permeability from the Schlumberger Oilfield Glossary.
The term “permeability” is defined as                “The ability, or measurement of a rock's ability, to transmit fluids, typically measured in darcies or millidarcies. Formations that transmit fluids readily, such as sandstones, are described as permeable and tend to have many large, well-connected pores. Impermeable formations, such as shales and siltstones, tend to be finer grained or of a mixed grain size, with smaller, fewer, or less interconnected pores. Absolute permeability is the measurement of the permeability conducted when a single fluid, or phase, is present in the rock.”        
The term “effective permeability” is defined as “The ability to preferentially flow or transmit a particular fluid when other immiscible fluids are present in the reservoir (e.g., effective permeability of gas in a gas-water reservoir).” Thus, a permeability measuring device would be measuring the effective permeability of a hydrocarbon in a situation such as that shown in FIG. 5, where there is water coating the grains due to the effect of invasion. The presence of the water around the matrix grains can have a large effect in reducing the effective permeability of tight gas sands.
The change in effective permeability can also have a significant effect on reservoir testing and evaluation. One type of post-drilling test involves producing fluid from the reservoir, shutting-in the well, collecting samples with a probe or dual packers, reducing pressure in a test volume and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different depths or point within a single reservoir and/or at several different reservoirs within a given borehole. One of the important aspects of the data collected during such a test is the pressure build-up information gathered after drawing the pressure down. From these data, information can be derived as to permeability, and size of the reservoir. Further, actual samples of the reservoir fluid must be obtained, and these samples must be tested to gather Pressure-Volume-Temperature and fluid properties such as density, viscosity and composition.
In order to perform these important tests, some systems require retrieval of the drill string from the borehole. Thereafter, a different tool, designed for the testing, is run into the borehole. A wireline is often used to lower the test tool into the borehole. The test tool sometimes utilizes packers for isolating the reservoir. Numerous communication devices have been designed which provide for manipulation of the test assembly, or alternatively, provide for data transmission from the test assembly. Some of those designs include mud-pulse telemetry to or from a downhole microprocessor located within, or associated with the test assembly. Alternatively, a wire line can be lowered from the surface, into a landing receptacle located within a test assembly, establishing electrical signal communication between the surface and the test assembly. Regardless of the type of test equipment currently used, and regardless of the type of communication system used, the amount of time and money required for retrieving the drill string and running a second test rig into the hole is significant. Further, if the hole is highly deviated, a wire line can not be used to perform the testing, because the test tool may not enter the hole deep enough to reach the desired formation.
U.S. Pat. No. 5,803,186 to Berger et al and U.S. Pat. No. 6,609,568 to Krueger et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, disclose MWD systems that includes use of pressure and resistivity sensors with the MWD system, to allow for real time data transmission of those measurements. The devices disclosed in Berger and in Krueger allow obtaining static pressures, pressure build-ups, and pressure draw-downs with the work string, such as a drill string, in place. Also, computation of permeability and other reservoir parameters based on the pressure measurements can be accomplished without pulling the drill string.
Referring to FIG. 1, prior art methods typically include reducing pressure in a flow line that is in fluid communication with a borehole wall. In Step 2, a piston is used to increase the flow line volume thereby decreasing the flow line pressure. The rate of pressure decrease is such that formation fluid entering the flow line combines with fluid leaving the flow line to create a substantially linear pressure decrease. A “best straight line fit” is used to define a straight-line reference for a predetermined acceptable deviation determination. The acceptable deviation shown is 2σ from the straight line. Once the straight-line reference is determined, the volume increase is maintained at a steady rate. At a time t1, the pressure exceeds the 2σ limit and it is assumed that the flow line pressure being below the formation pressure causes the deviation. At t1, the drawdown is discontinued and the pressure is allowed to stabilize in Step 3. At t2, another drawdown cycle is started which may include using a new straight-line reference. The drawdown cycle is repeated until the flow line stabilizes at a pressure twice. Step 5 starts at t4 and shows a final drawdown cycle for determining permeability of the formation. Step 5 ends at t5 when the flow line pressure builds up to the borehole pressure Pm. With the flow line pressure equalized to the borehole pressure, the chance of sticking the tool is reduced. The tool can then be moved to a new test location or removed from the borehole. Methods for analyzing flow tests to estimate permeability are disclosed, for example, in U.S. Pat. No. 5,708,204 to Kasap, having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. Methods for analyzing flow tests in anisotropic formation to estimate horizontal and vertical permeabilities are disclosed in U.S. Pat. No. 7,448,263 to Sheng et al. and in U.S. Pat. No. 7,448,262 to Sheng et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference.
For reservoir development, the absolute permeability is of particular interest as it measures the ability of the hydrocarbons to flow into a well in the absence of other fluids. For this reason, measurements of effective permeability by prior art devices always underestimate the ability of a reservoir to produce hydrocarbons. The present disclosure is directed towards a method and apparatus for measuring a permeability that is closer to the absolute permeability than can be obtained with prior art devices, and with reducing the effect of formation damage.